1. Field of the Invention
The invention relates generally to apparatus for and methods of making asphaltic product, for example, asphalt paving material. More particularly, the invention relates to a combination of distinct pieces of equipment forming a production plant or system which is particularly adapted to utilize old asphaltic pavement comminuted as recycle aggregate in combination with other elements of asphalt paving material.
2. Discussion of the Prior Art
Improvements in the cleanliness of asphalt material production processes are items of constant concern to equipment manufacturers and to asphalt materials producers alike. In many instances, improvements leading to reduction in air pollution emanating from asphalt plants also tend to have detrimental effects on production volumes or other aspects of making the product.
For example, asphalt when heated above a certain temperature tends to vaporize. The vapor mixes with dust released during material drying operations and with hot drying gases. The vapor condensates when cooled and needs to be filtered out together with the dust before the drying gases are released into the atmosphere. If the concentration of hydrocarbon becomes too great, state of the art filters tend to clog and downtime of the equipment results.
In one known production process using recycle material, the burners generating hot drying gases for drying the recycle material may be adjusted to burn at low flame temperatures to minimize the generation of hydrocarbons from the recycled asphalt. However, the relatively low flame temperatures are described as causing the generation of carbon monoxide. To rid the exhaust of carbon monoxide and hydrocarbons the gases are routed from the recycle drier to a drying drum of virgin aggregate material and there by special channels into a full combustion flame of the drying drum for the virgin material. The drying apparatus for the virgin aggregate is then used to burn the carbon monoxide as well as to break up hydrocarbons that may have formed during the drying of the recycle asphalt.
In the referred to apparatus, disclosed in U.S. Pat. No. 4,705,404, the output aggregate materials of the low temperature recycle material drying drum and of the high temperature virgin material drying drum are than discharged and transferred to a pugmill type mixer. In the pugmill, the aggregate materials are mixed and are further combined with liquid asphalt, also referred to as asphalt cement and with fines to complete the asphalt product. Hydrocarbons released during the mixing operation in the pugmill are not captured or burned.
In other drum drying and mixing apparatus at least some of the exhaust gases from the mixing apparatus have been returned to the burner to eliminate hydrocarbons from the exhaust of the apparatus. Apparatus of the latter type is disclosed in U.S. Pat. No. 4,600,379, where the virgin aggregate material is dried in an inner of two concentric drums and mixing takes place in an outer of such concentric drums. The recycle aggregate material is added from an external supply directly to the outer of the two concentric drums.
In that the concentricity of the two drums place the mixing operation into proximity of the heating operation for the virgin material, the mixing temperature tends to approach the temperature of the virgin material.